The present invention relates to a belt cleaning apparatus for cleaning a surface of a belt which is stretched between a plurality of rollers so as to be circularly movable, and more particularly to a belt cleaning apparatus for cleaning a surface of a photoconductive belt or conveyor belt which is stretched between a plurality of rollers so as to be circularly movable and which is prevented from moving in its both side directions by elongated both side-stop members. In addition, the present invention relates to an image forming apparatus including such a belt cleaning apparatus.
Conventionally, sheet conveyor belts are used in electrophotographic type image forming apparatuses such as printers and copying machines. FIGS. 5A and 5B show an example of a belt cleaning apparatus for cleaning a surface of such sheet conveyor belt, and FIG. 6 shows an arrangement of a main part of an image forming apparatus, the apparatus furnishing with the conventional belt cleaning apparatus. As shown in FIG. 6, the image forming apparatus comprises a photoconductive drum 1 which is rotatable in a clockwise direction, and further comprises an initializing charger brush 2, a writing head 3, a developing unit 4, a transfer brush 5, and a drum cleaner 6, those of which being arranged along a peripheral surface of the drum 1.
Generally, the photoconductive drum 1 is formed by uniformly depositing a photoconductor on a peripheral surface of an electrically conductive metallic roller to form a photoconductive layer on the surface. The initializing charger brush 2 applies, for example, a high negative voltage to the drum 1, thereby uniformly charging the photoconductive layer on the peripheral surface of the drum 1 to a high negative electric potential. The writing head 3 has a laser or LED light source, and selectively exposes the highly negatively charged peripheral surface of the drum 1 in accordance with image information supplied to the head, to form a negative low-potential section with a lowered electric potential. As a result, an electrostatic latent image is formed by the low-electric potential section in the aforesaid highly negatively charged peripheral surface of the drum 1.
The developing unit 4 contains therein a non-magnetic toner 4-1, and supports a developing roller 4-2 in its bottom opening. The roller 4-2 faces the photoconductive drum 1, and rotates with carrying on its peripheral surface a thin layer of the toner 4-1 that is charged to a low negative electric potential generated by rotational friction between the toner and the roller. In a region where the developing roller 4-2 and the drum 1 face each other, the electric potential of the low-electric potential section of the static image on the drum 1 is relatively high compared with that of the toner 4-1 on the roller 4-2. By an electric field generated by this electric potential difference, the negatively charged nonmagnetic toner 4-1 is transferred to the low-electric potential section on the photoconductive drum 1. Thereupon, a toner image is formed on the drum 1 (that is, the latent image on the drum 1 is reversely developed). As the drum 1 rotates, this toner image gradually moves to a region where the drum 1 and the transfer brush 5 face each other. A paper sheet 7 to which the toner image on the drum 1 is to be transferred, is transported to the facing region between the drum 1 and the brush 5.
Conventionally, it is very usual to employ a plurality of transportation roller pairs and transportation guide plates to transport the sheet 7. These days, however, a conveyor belt is frequently used for this purpose because it can steadily transport the sheet in a high-speed image forming process. The conveyor belt system is advantageous in a case that a plurality of photoconductive drums are arranged side by side to form a plurality of image forming units corresponding to various colors for full-color image formation, because it is easy to arrange the photoconductive drums side by side along the conveyer belt. In general, the conveyor belt 8 is formed of a dielectric or semiconductor film. The belt 8 is a horizontally stretched loop, horizontally opposite end portions of which are supported by a rotating roller 9 and a driven roller 10, individually. An upper surface of an upper extending portion of the belt 8 is in contact with a lower end of the peripheral surface of the photoconductive drum 1, and the belt 8 is moved circularly in a counterclockwise direction indicated by an arrow "a" in FIG. 6. The sheet 7 is placed on the upper surface of the upper extending portion, and is transported to the facing region between the drum 1 and the transfer brush 5 in such a manner that it is held together with the belt 8 by the driven roller 10 and an auxiliary roller 11. By the way, it is well known to apply an attraction bias voltage of a predetermined polarity to the auxiliary roller 11 from a power source (not shown). According to this, the sheet 7 is attracted on the belt 8 by the attraction bias voltage and is further pressed on the belt 8 by the auxiliary roller 11, so that the sheet 7 can be transported more securely by means of the conveyor belt 8.
The transfer brush 5 faces the photoconductive drum 1 with the conveyor belt 8 interposed there-between, and slidably contacts a back surface of the upper extending portion of the belt 8, thus forming a transfer section. The brush 5 is formed of an electrically conductive brush-shaped member, and is connected to a positive power source (not shown). The transfer brush 5 applies a positive transfer bias to the sheet 7 through the conveyor belt 8. The negative toner image on the drum 1 is transferred to the sheet 7 having a positive potential. The sheet 7 on which the toner image have been transferred, is separated from the upper extending portion of the belt 8 at its downstream end with respect to the transportation direction. Then, the toner image is thermally fixed on the sheet 7 by means of a fixing unit (not shown), and is discharged from the image forming apparatus.
Since the attraction and transfer bias voltages are applied to the conveyor belt 8, the toner, paper dust, etc. are easily attached to those regions of the conveyor belt 8 on which the sheet 7 is not placed. To remove the attached toner, paper dust, etc. from the conveyor belt 8, a belt cleaning apparatus is provided under the conveyor belt 8. The apparatus includes a blade scraper 12 and a cleaner bottle 13. The scraper 12 is pressed against a lower surface of a lower extending portion of the belt 8 at a position corresponding to the driven roller 10, to scrape off the attached toner, paper dust, etc. on the belt 8. The bottle 13 receives and stores the toner, dust, etc. removed from the belt 8 in this manner.
FIG. 5A is a cross sectional view showing the belt cleaning apparatus which includes the conveyor belt 8, the driven roller 10, the blade scraper 12, and the cleaner bottle 13, and FIG. 5B is an enlarged cross sectional view showing a portion encircled by a broken-line VB on a left end side of the cleaning apparatus in FIG. 5A. The cleaning apparatus shown in FIG. 5A has a symmetrical structure in its cross section so that a right end of the cleaning apparatus in FIG. 5A has the same structure as that of the left end of the cleaning apparatus in FIG. 5A. As shown in FIG. 5A, an upper end portion of the blade scraper 12 is pressed against the lower surface of the lower extending portion of the conveyor belt 8, while a lower end portion thereof is supported by a support member 14. As shown in FIG. 6, that portion of the belt 8 on which the scraper 12 is pressed against is supported by the driven roller 10. Thus, the roller 10 further serves as a backup roller for the scraper 12, so that the belt 8 will not escape from the upper end portion of the scraper 12 while the upper end portion of the scraper 12 is pressed on the belt 8. In this manner, the scraper 12 can be pressed against the belt 8 securely enough to remove the toner, dust, etc. from the belt 8. The driven roller 10 is supported on a frame 16 of the image forming apparatus by means of a support shaft 15.
Generally, a belt conveyor system is provided with a side-stopper mechanism for preventing the conveyor belt from sideslipping in its width direction. This mechanism includes a pair of elastic side-stopper members 17 which are fixed to both side end portions of the back or inner surface of the conveyor belt 8 shown in FIG. 5A and extend in a longitudinal direction of the belt 8.
Since these side-stopper members 17 are designed to abut the side surfaces of the driven roller 10 and the rotating roller 9 to prevent the conveyor belt 8 from sideslipping, the both side end portions of the back or inner surface of the belt on which the side stopper members 17 are fixed are not supported by those rollers 9 and 10. Thus, when the both side end portions of the belt 8 reach the upper end portion of the blade scraper 12, the both side end portions easily bend to move away from the upper end portion of the scraper 12 as shown in FIG. 5B, so that the scraper 12 cannot be pressed against the both side end portions of an outer surface of the lower extending portion of the conveyor belt 8 enough to remove the toner, dust, etc. from the both side end portions of the outer surface of the belt 8. The toner, dust, etc. remaining on the both side end portions of the outer surface of the belt 8 are finally moved to a central portion of the outer surface of the belt 8, and possibly produce an unfavorable result in the image transfer operation. This problem cannot be very serious if the width of the belt 8 is made greater enough than that of maximum-width paper sheets (that is, a width of the photoconductive drum or a width of the developing unit) usable in the image forming apparatus. If this is done, however, the whole size of the apparatus inevitably becomes large as compared with the maximum-width paper sheet. Accordingly, the above described problem becomes serious to minimize the whole size of the apparatus as small as possible compared to the maximum-width paper sheet usable in the apparatus.
If the upper extending portion of the conveyor belt 8 free from the press of the blade scraper 12 is subjected to a leftward deflective stress in its width direction in FIG. 7A, for example, while the belt 8 is circulating, the belt 8 easily deflects leftward as indicated by an arrow "b" in FIG. 7B despite the provision of the aforesaid side-stopper members. This deflection or sideslipping often causes the right-hand side-stopper member 17 to run onto the peripheral surface of each of the rotating roller 9, the driven roller 10, and tension rollers (not shown in FIG. 6) arranged between the rollers 9 and 10. Since the stopper member 17, like the conveyor belt 8, is an endless continuous structure, once the stopper member 17 runs onto the peripheral surface of each of the rollers, this running of the stopper member 17 on the peripheral surface continues endlessly.
In order to solve this problem, the following arrangement has been proposed. In this arrangement, as shown in FIG. 7C, a pair of belt retaining mechanisms are provided on both opposite ends of a shaft of a roller 18, the roller being the tension roller, the rotating roller 9, or the driven roller 10, for pressing the belt 8 toward its a center in its width direction. Each of the mechanisms includes a retaining flange 19 axially movably mounted on each end portion of the shaft of the roller 18, a spring 21 coaxially mounted on each end portion of the shaft, for urging the flange 19 toward the corresponding side edge of the belt 8, and a nut 22 screwed on each end portion of the shaft, for preventing removal of the spring 21 and adjusting the urging force of the spring. In this arrangement, if the urging force of the springs 21 are properly adjusted, the deflecting or the sideslipping of the belt 8 can be considerably reduced.
This arrangement, however, requires more components than those used in the arrangement of FIG. 7A, and therefore, makes inventory control more troublesome than that of the arrangement of FIG. 7A. Further, since this arrangement increases the number of assembly steps, a manufacturing cost of this arrangement becomes high. Further, the belt rotating mechanisms provided on the both end portions of the shaft of the roller 18 hinder the reduction of the whole size of a body of an apparatus, the apparatus using the belt 8. Furthermore, since the both side edges of the belt 8 are pressed inward in this arrangement, the cross section of the belt 8 is liable to bend upwardly or downwardly, and this bending may possibly result in unsuccessful image transfer.